Have you checked out these guys?

Hire for Baby | Baby Equipment Store | Baby Feeding Equipment | Car Seat Hire | Hire Baby Capsule |

I have used them before and not found them too expensive, but not for car seats sorry! Otherwise, could you hire them for a couple of weeks and check out some local baby and kid markets for some second hand ones? Generally second hand ones are not recommended though as you can never be certain that they have not been in an accident.

Good luck finding some!